When nature calls, either slip away quietly or excuse yourself from the group. Leave it clean, always flush and never discuss.
Generally, loo is the preferred term, 'Ladies' or 'Gents' in public venues is also widely used. 'Lavatory' is unambiguous, while 'Toilet' is the internationally recognised word, but may still raise an eyebrow in more class-conscious circles.
The British have a range of euphemisms and circumlocutions for the loo: including 'bog' (vulgar and masculine), 'little girls' room' (embarrassingly coy), 'powdering one's nose' (equally coy) and 'public conveniences' (a bureaucratic evasion). 'Spending a penny' is an archaic term that harks back to the days when public 'conveniences' were operated by a penny slot machine in the door. All of these terms are best avoided.
In some clubs, hotels, restaurants, railway stations and department stores, you will encounter a loo attendant - always leave some small change in their tip bowl.